Ballast water is seawater or the like carried in a ship to provide safe voyage in the sea when the ship carries no cargo. Ballast water is taken up from a seawater environment around a ship at a departure port of the ship and loaded into the ship. The ballast water is discharged around the ship at a destination port of the ship.
The ballast water is discharged into a seawater environment different from a seawater environment where the ballast water is taken up. Organisms in the ballast water are transported to a seawater environment different from their original habitat, resulting in a problem in that the organisms in the ballast water affect the ecosystem in the seawater environment into which the ballast water is discharged.
In order to solve this problem, various methods for performing a purification treatment of ballast water have been developed. For example, Japanese Unexamined Patent Application Publication No. 2013-23187 (PTL 1) discloses a ballast water treatment apparatus for the purpose of reliably and efficiently conducting a treatment for rendering ballast water harmless. This treatment apparatus includes a filter and an ultraviolet irradiation unit. Untreated water introduced into a ballast water treatment line is first allowed to pass through the filter. Next, the untreated water passes through the ultraviolet irradiation unit. While the untreated water passes through the ultraviolet irradiation unit, the untreated water is irradiated with ultraviolet light. Consequently, the untreated water is sterilized.
The above apparatus is provided in order to solve a problem in that the amount of ultraviolet irradiation is insufficient in a state where untreated water has a very high turbidity. Specifically, the treatment apparatus disclosed in the above document discharges pumped untreated water around a ship without allowing the untreated water to pass through the ultraviolet irradiation unit until the illuminance of an ultraviolet lamp reaches a predetermined value. When the illuminance of the ultraviolet lamp reaches the predetermined value, the pumped untreated water is allowed to pass through the ultraviolet irradiation unit and poured into a ballast tank.